1. Field of the Invention
The present invention relates to the field of depollution of the exhaust gas notably discharged by vehicles equipped with spark-ignition engines.
2. Description of the Prior Art
In most industrialized countries, increasingly stringent standards are applied as regarding atmospheric pollutant discharges by motor vehicles. The regulated pollutants are notably carbon oxide (CO), nitrogen oxides (NO.sub.x) and various unburned hydrocarbons.
Well-known depollution systems can be redox catalysts such as three-way catalysts placed in the exhaust line of vehicles. The problem posed by these systems is that they are effective only from a given exhaust gas temperature, generally of the order of 300.degree. C.
Under cold conditions, most of the well-known catalysts thus do not remove all of the pollutants and notably the totally or partly unburned hydrocarbons which are therefore discharged in great amounts to the atmosphere. In fact, it is well-known that more than 80% of the unburned hydrocarbons are discharged during the first two minutes of a standard emissions discharge evaluation cycle.
Various means are thus investigated to eliminate the pollutants under cold conditions, i.e. from the starting of the vehicle to the time when the exhaust gases reach about 300.degree. C., a time when conventional catalysts are initiated.
Patent application EP-A1-0,691,459 discloses a depollution system comprising a three-way catalyst interposed between a hydrocarbon polymerization catalyst placed upstream and a hydrocarbon adsorber placed downstream. This system changes hydrocarbons of low molecular weight, i.e. of small size, into hydrocarbons of higher molecular weight. The latter are more readily trapped at the level of the hydrocarbon adsorber.
In document DE 4,326,121, an adsorbent is provided to "clean" the pollutants from the exhaust gases throughout the catalytic activity.
The problem with such systems is that they have no influence on the initiation of the three-way catalyst. They only allow either an increase the amount of hydrocarbons trapped for a given lapse of time, or prolonging the life of the catalyst. Nothing is provided in this prior art to solve the cold start-up problem when the exhaust cases are not hot enough to initiate the catalyst.
Also known are exhaust lines equipped with a first catalytic muffler, known as light-off muffler, placed in the exhaust line as close as possible to the engine, which is initiated at a first temperature of the exhaust gases at the engine outlet. A second catalytic muffler is placed behind the first one and is initiated at a second temperature above the first temperature.
Problems linked with light-off catalyst wear arise generally quite rapidly with such exhaust lines.
Besides, in well-known systems using a hydrocarbon adsorber, the latter is intended to trap the hydrocarbons while the exhaust gases become hot enough to initiate the conventional oxidation catalyst.